This invention relates generally to actuator mechanisms and more particularly to actuator mechanisms for positioning a rotary shaft in two rotary positions disposed approximately 180.degree. apart.
Actuator mechanisms are used to place devices into motion. One such device that utilizes an actuator is a magnetometer which measures the strength of a magnetic field. A magnetometer includes a core of magnetic material surrounded by a gating coil and a pickup coil which is driven by an alternating current source to periodically saturate and desaturate the magnetic core. The lines of flux of the ambient magnetic field passing through the core change the degree of saturation of the core produced by the magnetic field of the gating coil. Accordingly, the flux of the ambient magnetic field passing through the core induces a voltage which differs from the voltage induced by the gating coil as a function of the strength of the ambient magnetic field.
Magnetometers are sensitive to magnetic fields along the axis of the magnetic core and malfunction when different readings of the strength of the ambient magnetic field are obtained for positions of the core which are disposed about 180.degree. apart. When such different readings have been obtained, the magnetometer must be recalibrated by introducing a correction factor to ensure proper operation. The correction factor is obtained by subtracting the reading obtained at the two core positions disposed 180.degree. apart, dividing this quantity by 2 and then adding this quantity to the reading taken at the low reading position or, alternatively, subtracting it from the reading which is taken at the high reading.
Actuator mechanisms are used to rotate the magnetometer between the two positions so that readings of the magnetic field strength can be taken and compared to determine if the magnetometer is malfunctioning and needs calibration. Actuators used for rotating a shaft, and in particular a magnetometer, through angular displacements are well known. These actuators generally utilize one or more bimetallic springs which are wound in a convoluted configuration and are attached at their respective ends to the rotatable shaft and to a fixed point on a frame which rotatably supports the shaft. Application of heat to one of the bimetallic springs causes extension of the spring which rotates the shaft through an angular displacement. One disadvantage of these actuators is that once the heat is no longer applied to the spring, they begin to cool whereupon rotation of the shaft in the opposite direction occurs. There is no way of maintaining the shaft in the one position unless continuous heat having nonfluctuating temperature is applied.
Another disadvantage is that the surrounding environment may have a continuously changing temperature range which will effect the bimetal springs causing them to continuously bi-directionally rotate the magnetometer at different angular displacements when no rotation is desired.
Still another disadvantage is that the flexing life of bimetal springs is limited and the flexible characteristics of the bimetal springs degrade continually following each cycle of operation.
The afore-identified parent application disclosed an actuator which overcame the above and other disadvantages by utilizing a flexible spring whose flexibility is maintained over a large number of operational cycles and utilizes a locking mechanism to lock the shaft in the required rotary positions to counteract the temperature changes encounted and the removal of heat from the springs.
With the advent of interplanetary space exploration by manned and, in particularly, unmanned spacecraft a need arose for an actuator which will perform reliably over extremely long time periods as may be encountered by spacecraft traveling to a distant planet, such as Jupiter. In addition, since these actuators will encounter temperatures of -65.degree. C or less, in deep space they must be designed to be able to perform their function at these low temperatures. The prior art spring devices are incapable of functioning under these conditions for the reasons previously given. For example, the spring device disclosed in the parent application degrades in performance over extremely long time periods and its modulus of elasticity is only maintained down to a temperature of +10.degree. C. Whereupon the flexible characteristics required to rotate the shaft the desired angular displacement are not maintained.